(1) Field of the Invention
The present invention relates to an acousto-optic tunable filter capable of varying a selected wavelength, that performs selection of optical signals based on mode conversion using the acousto-optic effect, and in particular, relates to an acousto-optic tunable filter that achieves the flattening of filter characteristics.
(2) Related Art
For example, in wavelength division multiplexed (WDM) optical communications, in order to add or drop an optical signal of a required wavelength to or from a WDM signal light being propagated through an optical transmission path or the like, an optical add/drop device is used. For this optical add/drop device, there is known for example one which uses an arrayed waveguide grating (hereunder AWG), or one which uses an acousto-optic tunable filter (hereunder AOTF).
The conventional optical add/drop device using an AWG is basically of a fixed wavelength type configuration in which the wavelength of the optical signal to be added or dropped cannot be freely selected. Hence, this is only applicable to fixed networks, and is difficult to be applied to networks where expansion or modification of the optical line is frequently carried out. Therefore, there is also proposed an optical add/drop device where the wavelength is selectable by combining an AWG and an optical switch. However, in such an optical add/drop device where an AWG and an optical switch are combined, there is the disadvantage in that construction becomes complex resulting in higher cost.
On the other hand, in the conventional optical add/drop device using the AOTF, the construction is simple giving a low cost. This AOTF has the advantage in that for example an optical signal of a 1.5 xcexcm band can be selected based on the application of an electrical signal of 170 MHz band, and moreover, by applying an electrical signal of a plurality of frequencies at once, optical signals of a plurality of wavelengths can be simultaneously selected.
However, in the conventional optical add/drop device using the AOTF, there is a problem attributable to the filter characteristics of the AOTF. That is, considering a case where an optical signal of a required wavelength is selected to be branched from an input light by an optical branching device that uses the AOTF shown in (A) of FIG. 17, the typical AOTF has filter characteristics in that, in the vicinity of a wavelength which coincides with a wavelength of optical signal to be selected (hereunder the selected center wavelength), the branching characteristic (the transmissivity of the branched light) relative to the wavelength is changed steeply as shown in (B) of FIG. 17. On the other hand, for the optical signals of respective wavelengths included in the input light to the AOTF, as shown in (C) of FIG. 17, the spectra thereof spread slightly from the center wavelength due for example to an influence of modulation. Therefore, in the spectrum of the optical signal branched by the AOTF, as shown in (D) of FIG. 17, the light power drops on the short wavelength side and the long wavelength side of the center wavelength. In optical transmission systems constructed using an optical branching device having such a characteristic, there is a problem of the likelihood of an increase in line errors and the like.
To solve the problem attributable to the above AOTF filter characteristics, there is proposed in the article a technique for flattening the AOTF filter characteristics; Janet L. Jackel et al., xe2x80x9cA Passband-flattened Acousto-optic Filterxe2x80x9d, IEEE Photonics Technology Letters, Vol. 7 No. 3, pp 318-320 1995. In the technique described in this article, for a directional coupling type AOTF, in intensity function of a surface acoustic wave (hereunder SAW) capable of flattening the filter characteristics is predicted based on an approximation by combining two half-period sine waves, and a SAW drive method is proposed in accordance with this intensity function.
However, in the above-mentioned conventional technique, there is a disadvantage in that the width of wavelength band to be flattened is comparatively wide. More specifically, it is reported that the band width flattened to a fluctuation amount of 0.5 dB or less becomes around 1 nm. In recent WDM optical communications, however, technological development is advancing for wavelength division multiplexing optical signals at wave length intervals of for example 0.8 nm (100 GHz) or 0.4 nm (50 GHz), to transmit the thus wavelength division multiplexed light. For a WDM signal light of such narrow wavelength intervals, it is difficult to construct an optical add/drop device or the like by applying the conventional AOTF having such a wide bandwidth.
Furthermore, in the conventional AOTF, in order to realize a SAW in accordance with an intensity function capable of flattening the filter characteristics, a SAW generated from a single electrode must be propagated over a long distance along an optical waveguide. Hence there is also the disadvantage in that the overall length of the AOTF becomes long.
The present invention has been achieved in view of the above mentioned problems, and it is an object of the present invention to provide an acousto-optic tunable filter that realizes filter characteristics where the wavelength characteristics in the vicinity of selected center wavelength are flat, and the bandwidth of the selected wavelength is sufficiently narrow.
In order to achieve the above object, an AOTF of the present invention is constituted such that a plurality of areas each provided with an optical filter configuration capable of varying a selected wavelength, for performing selection of optical signals based on mode conversion using the acousto-optic effect, are respectively connected via a mode branching device, wherein at least one area of the plurality of areas functions as a mode coupling section that mode converts an optical signal corresponding to the selected wavelength, and at least one of other areas functions as a wavelength characteristic flattening section that again mode converts only the optical component of a part of the selected optical signal mode converted by the mode coupling section in the vicinity of a center wavelength thereof. Then, the mode branching device connected to an output side of the wavelength characteristic flattening section, branches the selected optical signal except for the optical component mode converted by the wavelength characteristic flattening section, to output the branched selected optical signal, to thereby perform flattening of the wavelength characteristics in the vicinity of the center wavelength of the selected optical signal.
In the AOTF of such a configuration, input optical signals are sent to the mode coupling section, wherein the optical signal corresponding to the selected wavelength is converted from a TE mode into a TM mode (or from the TM mode into the TE mode), and an output light of the mode coupling section passes through the mode branching device at a latter stage, so that the mode converted optical signal is branched, to be sent to the wavelength characteristic flattening section as the selected optical signal. In the wavelength characteristic flattening section, only the optical component of a part of the selected optical signal in the vicinity of the center wavelength thereof is again converted from the TM mode into the TE mode (or from the TE mode into the TM mode), and the output light of the wavelength characteristic flattening section passes through the mode branching device at the latter stage, so that the optical component that has not been subjected to second mode conversion in the wavelength characteristic flattening section is branched, and the selected optical signal of which light power in vicinity of the center wavelength has been flattened is output.
Moreover, as one aspect of the AOTF, the configuration may be such that the mode coupling section propagates therethrough a surface acoustic wave having a frequency corresponding to the selected wavelength and having the intensity capable of mode converting the optical signal corresponding to the frequency, along an optical waveguide, and also, the wavelength characteristic flattening section propagates therethrough a surface acoustic wave having a frequency corresponding to the selected wavelength and having the intensity smaller than the intensity of the surface acoustic wave propagated within said mode coupling section, along the optical waveguide.
With the AOTF of such a configuration, in the wavelength characteristic flattening section, flattening of the selected optical signal is performed in the vicinity of the center wavelength, by applying the surface acoustic wave with the intensity smaller than that of the surface acoustic wave applied at the mode coupling section.
Furthermore, as a specific configuration of the AOTF, the mode coupling section and the wavelength characteristic flattening section each may have an electrode that generates the surface acoustic wave by applying an electrical signal, a guide that propagates the surface acoustic wave from said electrode along the optical waveguide, and an absorber that absorbs to terminate the surface acoustic wave being propagated through the guide. With such a configuration, in the respective electrodes of the mode coupling section and the wavelength characteristic flattening section, surface acoustic waves of which frequencies are the same but intensities are different, are respectively generated, and the surface acoustic waves pass through the guides to be propagated to the absorbers, respectively.
Alternatively, the mode coupling section may have an electrode that generates the surface acoustic wave by applying an electrical signal, a guide that propagates the surface acoustic wave from the electrode along the optical waveguide, and an absorber that attenuates the surface acoustic wave being propagated through the guide, and then transmit the attenuated surface acoustic wave to the wavelength characteristic flattening section. The wavelength characteristic flattening section may have a guide that propagates the attenuated surface acoustic wave transmitted from the absorber of the mode coupling section along the optical waveguide, and an absorber that absorbs to terminate the surface acoustic wave being propagated through the guide. With such a configuration, the surface acoustic wave generated by the electrode of the mode coupling section passes through the guide to be propagated to the absorber, and then the surface acoustic wave that has passed through the absorber to be attenuated to the required intensity passes through the guide of the wavelength characteristic flattening section to be propagated to the absorber. According to this configuration, it is not necessary to provide the electrode for generating the surface acoustic wave in the wavelength characteristic flattening section, and hence simplification of the construction can be achieved.
Furthermore, as another aspect of the aforementioned AOTF, the mode coupling section may be constituted such that the surface acoustic wave having the frequency corresponding to the selected wavelength is propagated, along the optical waveguide over a predetermined interference length capable of mode converting the optical signal corresponding to the frequency. And the wavelength characteristic flattening section may be constituted such that the surface acoustic wave having the frequency corresponding to the selected wavelength is propagated, along the optical waveguide over an interference length different to the predetermined interference length.
In the AOTF of this configuration, the flattening of the selected optical signal in the vicinity of the center wavelength thereof is performed by setting the interference length of the optical signal and the surface acoustic wave in the wavelength characteristic flattening section to be shifted from an optimum interference length capable of mode converting all the components of the selected optical signal. As a result, the same surface acoustic waves can be applied to the mode coupling section and the wavelength characteristic flattening section, and hence adjustment of the surface acoustic wave for each section is practically unnecessary.
Other objects, characteristics and advantages of the present invention will become apparent from the following description of embodiments, in association with the appended drawings.